Deans announced the controversial elements of his squad selection on Sunday, ruling out the inclusion of fly-half Quade Cooper and welcoming Kurtley Beale back to the fold despite his battle with alcohol-related issues.
The other players included in the squad in addition to the 25 initially picked last month were lock Hugh McMeniman, fit-again winger Digby Ioane, centre Pat McCabe and utility forward Peter Kimlin.
Deans said the decision to pick Beale was helped by his versatility and made in agreement with his counsellors, whose work will continue while he is in the squad.
"He showed that as recently as last year's spring tour when he performed outstandingly and led the team from fly-half in adverse circumstances against many of the opposition players we now face again," Deans said.
"His work last week during the first camp supported our contention that it was not too soon for him to get started again, from either a physical or a mental stand point.
"His physical state is a testament, both to how hard he has worked, but also his desire to get started again."
Queensland Reds lock Horwill has held the captaincy when fit since assuming the role in a shock decision made on the eve on the 2011 World Cup, and his resumption of the role after missing last year's international season was no surprise.
The inclusion of Sio in the squad is more of a shock, given the quality and experience of the Lions props and that the 21-year-old is still in his first full season of Super Rugby with the ACT Brumbies.
"Scott has had a remarkable year," Deans said of the Sydney-born son of Samoa prop Tevita Sio.
"While he is still developing his game, his work rate around the field, impact in contact and in defence, and the raw power he brings to his set piece indicated to us that he was ready to get underway."
Left winger Ioane, who would have been in the original party but for injury, was rated "touch and go" by the Wallabies for the Test series after an operation on his knee last month.
"Ioane's attendance at last week's camp allowed medical staff to assess the progress on the rehabilitation of his right knee following surgery, clearing the way for his involvement, with a return date to be determined as his training load increases," read the statement.
Hard-running, big-tackling McCabe has enjoyed more than his fair share of injuries over the last couple of years but has also been a Deans favourite at inside centre when fit.
McMeniman, who can also play at flanker, was more of a bolter, having won the last of his 21 caps in 2008, while Kimlin, without a cap since his second appearance in 2009, is rewarded for his ability to play at lock, blindside flanker and number eight.
The loss of Sitaleki Timani and Scott Higginbotham to injury left Australia light on bigger forwards to populate the back two rows of the scrum, while scrum-half Nic White's shoulder injury left Nick Phipps unchallenged as understudy to Will Genia.
Luke Burgess, fresh off the plane after two years in France, will train with the squad, however, and could get a chance to add to his 37 caps if there is an injury to either of the half-backs.
Another player training but not technically part of the squad is hooker James Hanson, who played for Queensland Reds in last Saturday's 22-12 defeat to the Lions in Brisbane.
"Certainly our recent experience with injury highlights the need to prepare with extensive cover to ensure relatively seamless change should it be required," Deans said.
The Lions return to Brisbane to face the Wallabies on June 22 with further Tests in Melbourne and Sydney to follow.
Backs: Kurtley Beale, Berrick Barnes, Israel Folau, Digby Ioane, Nick Cummins, Joe Tomane, Adam Ashley-Cooper, Rob Horne, Pat McCabe, Christian Leali'ifano, James O'Connor, Will Genia, Nick Phipps
Forwards: Wycliff Palu, Ben McCalman, Ben Mowen, Michael Hooper, Liam Gill, Dave Dennis, Peter Kimlin, Hugh McMeniman, James Horwill (captain), Rob Simmons, Kane Douglas, Ben Alexander, Sekope Kepu, James Slipper, Benn Robinson, Scott Sio, Stephen Moore, Saia Faingaa
- Sports & Recreation
- Kurtley Beale
- Digby Ioane
- Hugh McMeniman
- Peter Kimlin